[Makefile] `make build` always builds a debug and a prod version

My PC takes a long time for building the desktop client. For each branch I work with, I use the adjustments of !179 (closed)

For me it would be nice to get this merged somehow. But I can also live with the current situation. I don't fully understand the build process and for me it was some trial and error.

The other thing is: make build currently builds two versions: one with debug symbols and one stripped for prod deployment. This is configured in bitmask.pro (I think). We don't need the debug version for 99%, but it takes unnecessary time to build. For me it would be nice if

• the default behavior is: build the only the prod version
• have an environment variable to build a debug version

Any thoughts on this @jkito?